The operating environment within a gas turbine engine is both thermally and chemically hostile. While significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, improvements in the environmental properties of such alloys are often achieved at the expense of mechanical properties, and vice versa. Accordingly, components formed from superalloys whose chemistries are formulated to have optimum mechanical properties at high temperatures are often susceptible to environmental attack, especially if used in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to provide such components with a protective environmental coating that inhibits oxidation and hot corrosion.
Coating materials that have found wide use for this purpose include diffusion aluminide coatings, which are generally single-layer oxidation-resistant layers formed by diffusion processes, such as pack cementation or vapor phase deposition. Diffusion aluminiding processes generally entail reacting the surface of a component with an aluminum-containing gas composition to form two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MA1, where M is iron, nickel or cobalt, depending on the superalloy substrate material. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. During high temperature exposure in air, the MAl intermetallic forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the diffusion coating and the underlying substrate.
Vapor phase deposition processes used to form diffusion aluminide coatings (known as vapor phase aluminiding, or VPA) generally involve the use of an aluminum-rich source (donor) material composed of aluminum or an aluminum alloy that is mixed or bonded with a metal having a higher melting temperature. Primary examples for the higher melting constituent include chromium, cobalt and iron. The donor material is typically in particulate form, with particle sizes typically on the order of about five to twenty millimeters in diameter. The donor particles and a suitable halide salt activator, such as NH.sub.4 F, NaF, KF, NH.sub.4 Cl or AlF.sub.3, are then heated to a temperature that will vaporize the activator, which reacts with the donor material, thereby forming a volatile aluminum halide that reacts at the surface of the component to form the diffusion aluminide coating. The chromium, cobalt or iron constituent of the donor does not deposit on the component, but instead merely serves as an inert carrier or binder for the aluminum.
At the end of the coating process, an aluminum-depleted layer is present on the surfaces of the donor particles. Over multiple coating operations, this layer becomes an encapsulating shell composed predominantly of the high-temperature constituent of the donor, and inhibits further removal of aluminum from the donor particles. In the past, used donor material has been processed through a sieve sizing operation to remove the particle shells and undersized particles, permitting reuse of the donor material. However, shell removal is incomplete, with the result that the donor material does not perform as well as when new. As an example, the time required to deposit an aluminide coating of desired thickness is often significantly longer than would otherwise be expected. Accordingly, though a potential cost advantage exists, there are significant process limits to the use of aluminum alloy donor material reclaimed from vapor phase aluminiding processes.
From the above, it can be appreciated that it would be desirable if a process were available to improve the quality of aluminum alloy donor material reclaimed from vapor phase aluminiding processes, so that the consistency and uniformity of diffusion aluminide coating produced by the reclaimed donor material might also be improved.